As is well known in the mining art, in an underground mine, the roof must be supported at spaced intervals in order to assure the safety of the miners. For many years, this support was provided by shoring timbers positioned at spaced locations longitudinally along the mine tunnels. Shoring timbers have in recent years been almost totally replaced by modern rock bolt assemblies. These devices are inserted in bore holes along the rock face and serve to fasten the adjacent layers of rock together, thus preventing cave-ins. The safety of mines has improved considerably since the introduction and continued development of the modern rock bolt.
Heretofore, the rock bolt has been set in the bore hole and the rod or bolt simply tightened to a predetermined torque designed to support the strata of rock forming the rock face. The tension in the rod is usually gauged so that a safety factor is provided, i.e., the bolt assembly is tightened to a point below the elastic limit of the rod. If a serious fault develops in the rock, the tension increases and thereafter this bolt assembly imposes a safety hazard since failure may then occur at any time.
To alleviate the danger, there has been considerable development in the field of tension indicating devices, such as is shown in the U.S. Pat. No. to Cumming 3,133,468. These indicating devices help during the installation of the rock bolt assemblies, and are intended to alert mine personnel when the tension increases after installation. A spring washer or the like is flexed to varying degrees so the worker can determine when excessive tension is being approached.
These tension indicating devices are not designed for, and indeed are incapable of permitting, any more than minor rock face sagging. This is so, since the devices are merely spring devices with extremely limited travel. These prior art indicating devices also do not employ constant tension movement since mechanical springs are used. This latter feature means, that with a compression spring for example, the bolt is tensioned at a low setting equal only to some average spring force rating. When a fault occurs in the rock, the spring quickly bottoms out permitting the application of excessive force directly to the support rod. When the design force is exceeded in this way, failure inevitably occurs, just as in prior art rock bolt assemblies without indicating devices, unless the inspector is lucky enough to notice the change and has replaced the bolt before another rock shift occurs.
From this background, I have recognized a need for providing a rock bolt that allows reasonable rock sag in a mine and does so while maintaining a constant tension in the rod or bolt. The constant tension feature would be self-adjusting and thus prevent the elastic limit of the bolt from ever being reached. Extended travel is also necessary so that a rock fault within proven limits for a particular area may be accommodated. My theory is that if shifting of rock can occur while still maintaining the rock face in tact supported by the design tension, then cave-ins due to rock shifting can be virtually eliminated. This, in turn, will result in saving of lives, and it will also represent a considerable increase in efficiency since closing of mine shafts and clean up from wall failures will be minimized.